1. Field of the Invention
The present invention relates to methods and apparatus for welding, and more particularly concerns welding in which a controlled force is applied to press electrodes against a work piece.
2. Description of Related Art
In many types of welding it is necessary to press welding electrodes against a work piece while the welding operation takes place. Pressure welding of this type, in general, includes various types of resistance welding, thermo-compression welding, ultrasonic bonding and similar methods for joining materials. Materials to be joined include integrated circuit electrical connectors, IC chip leads and wire mesh connectors employed for electrically joining solar cells, among others.
Accordingly, the term "welding" as employed herein also includes various types of bonding which require application of controlled amounts of pressure during the bonding or joining operation.
Parallel-gap welding refers to a resistance welding operation employing a pair of mutually spaced electrodes which contact the work piece to provide flow of current from one electrode to the other through the work piece or work pieces against which the electrodes are pressed. The flow of electrical current through the work heats the work to a softened condition, and the simultaneous application of pressure provides strong mechanical bonding between elements of the work. Parallel-gap welding apparatus of the prior art include dead-weight weld heads wherein a weight applies the desired force. Open-loop weld heads may employ a solenoid to drive the weld head against the work, but the amount of current to drive the solenoid must be predetermined. Such devices provide no means for monitoring or otherwise precisely controlling the amount of force applied by the solenoid. Pneumatic and cable operation devices also fail to provide precise control of forces and add greater complexity to the apparatus.
In such prior devices the amount of force that is actually applied is not known, and therefore no history of force applied in a number of welds can be obtained to enable analysis and evaluation of reliability and consistency of the welds. Dead-weight weld heads are limited to vertical operation and, moreover, exhibit high inertia and impact forces which are undesirable.
Precise control of the amount of welding force and control of the profile of applied force are highly desirable, particularly in automatic welding equipment to improve repeatability, reliability, analysis and quality, but have not been available heretofore. Accordingly, it is an object of the present invention to provide for welding operations that avoid or eliminate above-mentioned problems.